English to Italian Science Chemistry; Chem Sci/Eng saldature e creep

Buongiorno a tutti

cosa sono le "double penetration butt welds"? Le butt welds sono le saldature continue, ma non ho mai sentito "a penetrazione doppia". Ecco la frase completa:

These effects have been demonstrated by Masuyama, who considered the effects of specimen size, shape and loading configuration in high chromium martensitic steels.25 It was reported that double penetration butt welds, ‘X’ type, perform worse than ‘U’ groove geometries for large-scale (30 mm2) uniaxial cross-weld specimens, and when tested in hot pressure vessels (seam welds subject to hoop stresses) for grade 122 steel at 650°C.